Objective:To study the effect and mechanism of Rhodobryum giganteum（Schwaegr.） Par.’s anti-atherosclerotic effect.Methods:Vascular endothelial cells were cultivated and the H

2

O

2

were used to induce the oxidative stress injury of human umbilical vascular endothelial cells（HUVEC）.Water extract of Rhodobryum giganteum（Schwaegr.） Par.was added to cultivated HUVEC

and the activity of the cells was carefully determined by OD value with MTT method.The Griess Reagent was used to detect the NO concentration of different groups.At the same time

the NO fluorescence analysis probe

DAF-FM DA（3-amino

4-aminomethyl-2′

7′-difluorescein

diacetate）

was used to determine the activity of NO synthase.Results:The most suitable stimulating concentration of H

2

O

2

on HUVEC is 12.5 mmol·L-1.Water extract of Rhodobryum giganteum（Schwaegr.）Par.was co-cultured with HUVEC damaged by H

2

O

2

.The OD values indicate that 3.33

2.50 mg·mL-1 water extract groups increase activity of the cells significantly compared with the model group（P

<

0.05）

and 3.33 mg·mL-1 is much better than 2.50 mg·mL-1 dose group.To determine the content of NO and NOS

the 2.50 mg·mL-1 dose has significant effect（P

<

0.05）.Conclusions: The H

2

O

2

concentration of 12.5 mmol·L-1 could be used to establish the injured model of endothelial cell successfully.2.50 and 3.33 mg·mL-1 water extract of Rhodobryum giganteum（Schwaegr.） Par.have protective effect on ECV304 injured by H

2

O

2

.2.50 mg·mL-1water extract of Rhodobryum giganteum（Schw aegr.） Par could increase the activity of NOS and promote the synthesis and secretion of NO.